Thailand's revered but ailing king urged his politically divided citizens to stand united for the "stability and security" of the country, after a week of protests calling for the overthrow of the government saw five people killed and more than 270 injured.

But the leader of the protests, Suthep Thaugsuban – a deputy premier in the previous administration – vowed that the fight would only end once the current "regime" was "eradicated" from Thailand, and told his supporters to prepare for another round of fighting on Friday.

The two sides had observed a two-day truce, beginning on Wednesday, out of respect for King Bhumibol Adulyadej's 86th birthday.

On Thursday the ailing monarch told a sea of yellow-clad onlookers, the colour of the monarchy: "Thailand has been peaceful for a long time because we are united and have worked together. Every Thai person should be aware of this and perform their role for the benefit of the country, which is the stability and security of the country." The gold-robed monarch, the longest reigning in the world, left hospital in August after a four-year stay, the result of various ailments, and spoke from the royal Klai Kanwon palace in Hua Hin, three hours south of Bangkok, where many yellow-shirted supporters had slept overnight – some on the streets – to celebrate his birthday with candlelit ceremonies and fireworks.

Although he has no official legal powers, Bhumibol is seen as the single unifying force in this nation of 67 million, with his pictures adorning many homes and shops and his birthday celebrated as both a national holiday and Father's Day. The king has previously stepped in during political deadlocks to ease tensions and many looked to his speech for clues on what direction, if any, the current deadlock might take now. But the short five-minute speech failed to address the current deadlock head on.

Suthep vowed to resume demonstrations until the embattled administration of Yingluck Shinawatra had been replaced by an unelected "people's council". "Today is a day that Thai people, nationwide, believe is an auspicious day," Suthep said from protest headquarters, the seized government complex in north Bangkok.

Protests led by Suthep over the past fortnight were instigated by an ill-conceived amnesty bill that would have allowed for the return of Yingluck's brother Thaksin, a divisive former PM who currently lives abroad in self-imposed exile to avoid a corruption conviction. Suthep considers the government to be part of a corrupt "Thaksin regime" that must be replaced with a royally appointed prime minister and cabinet, part of which would be manned by "good people" chosen from representatives of all segments of society. Critics have called Suthep's council utopian and absurd.

Analysts have warned, however, that the protest leader's plans – should they be abetted by any help from the military, judiciary or palace, which have stepped in in the past – could lead to civil war, with thousands of pro-Thaksin "red shirts" protesting against the so-called people's council, potentially leading to great bloodshed.

Political scientist Thitinan Pongsudhirak, of Thailand's Institute of Security and International Studies (ISIS), said the 64-year-old protest leader would be likely to forge ahead anyway, interpreting the king's speech for his own benefit.

"If Suthep maintains and intensifies his people's revolt, he would have taken the king's speech as the absence of a red light – unlike a similar speech in 2006, when the king chastised anti-government protesters for asking for an appointed government."

The leader of the red-shirt United Front of Democracy against Dictatorship (UDD), Jatupon Prompan, said a massive demonstration was already planned for Tuesday in Ayutthaya, roughly 55 miles from Bangkok, with roughly 10,000 red shirts expected to attend. "We will never give this land up to Suthep," Jatupon said.